To turn this into a velocity, you need to take into account the friction/viscosity of the fluid, and the shape of the thruster and nozzles.

Looking at photos of the Yamato prototype, it makes some sense that the nozzles would be tapered, so the velocity of the fluid at the inlet would be the same as the velocity of the boat, but the velocity at the output nozzle would be much higher (ie variable velocity along its length).

If you blocked the inlet, I expect that you would have less pressure at the inlet end than at the nozzle end - but it would take a very high voltage & current, and a very strong magnetic field to produce cavitation ("negative pressure") at the inlet. On the other hand, cavitation is a common problem on motor-driven propellers, causing erosion of the propeller blades. http://en.wikipedia.org/wiki/Cavitation

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